NO you cannot sit on a cloud.
A cloud really is just an illusion that makes it look like it's there but really the clouds in the sky are nothing more than tiny droplets of water suspended in the air, which refract the light and make them look white or grey and looks like a cloud.
So if you were to get to a cloud and try to sit on the cloud you would just fall through to the ground which would be deadly.
It would be nice if you could sit on a cloud but it's impossible.
Even touching a cloud is impossible because in the sky the cloud is just water droplets with light reflecting through them to make the appearance of a cloud from earth.